Soybean crops saw a significant increase in herbicide use.
Widespread adoption of genetically modified crops has reduced the use of insecticides, but increased the use of herbicides as weeds become more resistant, according to the largest study of GM crops and pesticide use to date.
Researchers studied annual data from more than 5,000 soybean and 5,000 maize farmers in the US from 1998 to 2011, far exceeding previous studies that have been limited to one or two years of data.
"We have repeated observations of the same farmers and can see when they adopted genetically modified seeds and how that changed their use of chemicals," said Federico Ciliberto from University of Virginia in the US.
Despite the decrease in insecticide use, continued growth in herbicide use poses a significant environmental problem as large doses of the chemicals can harm biodiversity and increase water and air pollution, researchers said.
Since 2008, genetically engineered crops have accounted for more than 80 per cent of maize and soybean crops planted in the US, they said.
Maize seeds are modified with two genes: one kills insects that eat the seed and other allows the seed to tolerate glyphosate, a herbicide commonly used in weed killers.
Soybeans are modified with just one glyphosate-resistant gene.
Unsurprisingly, maize farmers who used the insect-resistant seeds used significantly less insecticide - about 11.2 per cent less - than farmers who did not use genetically modified maize.
The maize farmers also used 1.3 per cent less herbicide over the 13-year period.
Soybean crops, on the other hand, saw a significant increase in herbicide use, with adopters of genetically modified crops using 28 per cent more herbicides than non-adopters, researchers said.
Mr Ciliberto, who carried the study alongside Edward D Perry of Kansas State University, David A Hennessy of Michigan State University and GianCarlo Moschini of Iowa State University, attributes this increase to the proliferation of glyphosate-resistant weeds.
"In the beginning, there was a reduction in herbicide use, but over time the use of chemicals increased because farmers were having to add new chemicals as weeds developed a resistance to glyphosate," Mr Ciliberto said.
Maize farmers, he said, have not yet had to address the same level of resistance, in part because they did not adopt genetically modified crops as quickly as their counterparts in the soy industry.
However, the study did find evidence that both maize and soybean farmers increased herbicide use during the last five years of the study, indicating that weed resistance is a growing problem for both groups.
"Evidence suggests that weeds are becoming more resistant and farmers are having to use additional chemicals, and more of them," Mr Ciliberto said.